Nepal has been devastated by the flash floods in Rasuwa district on the Indo-Tibetan border on Wednesday morning, after the waters breached the river banks. The huge surge of water sent down sediment and boulders through the Bhote Koshi and Trishuli river systems, raising concerns over further potential downstream impacts. About 270 persons are feared dead, a sizeable number being Indians, while around 1500 are still missing.
“According to local reports, water levels in the Bhote Koshi basin began rising rapidly around 9am local time on August 26. The flood breached riverbanks and travelled downstream through the Bhote Koshi River into the Trishuli River, with potential impacts extending into Nuwakot and Dhading districts,” says a report released by the Kathmandu-based International Centre for Integrated Mountain Development (ICIMOD), working on behalf of the people in eight countries of the Hindu Kush Himalaya (HKH), including India, Nepal and China. A copy of the report is with The Plurals.
“We are deeply saddened by this tragic disaster, which struck so suddenly and unexpectedly,” said Pema Gyamtsho, director-general of ICIMOD.
ICIMOD scientists are continuing to analyse hydrological, seismic, remote-sensing and field data to understand what caused the floods and if there are active risks. Though it is too early to know what role climate change has played in these floods, which could have been triggered by an ice-rock avalanche, scientists have always been warning about climate change altering glaciers, snow conditions, permafrost and mountain slopes across the HKH region. .
Ice-rock debris link to sudden surge
Scientists from ICIMOD and partner institutions in Nepal and China are trying to find out whether the flood could have been “triggered by an ice-rock avalanche originating from a high-altitude area”, said ICIMOD report.
“Initial assessment, based on video footage and local information suggest that a huge volume of ice and rock debris may have entered the Lende Khola, a tributary of the Bhote Koshi River”, says the report. “The event could have created a sudden surge of water that moved down sediment and large boulders downstream.
According to hydrological observations, the speed and magnitude of the flood wave were exceptional. Water levels on the Trishuli River at Galchchi reportedly rose 9 metres within 30 minutes, and at Malekhu by 7 metres in similar time”, it further explains.
Blockage might have led to avalanche
The floods reportedly damaged or destroyed several water monitoring stations along the Trishuli River. The Galchchi monitoring station remains operational. A number of hydropower facilities are located along the Bhote Koshi River and connected river systems. Many of these have been affected.
A concern for experts is finding out if debris temporarily blocked the river at a narrow section and formed a landslide dam. “Such blockage can create secondary hazard if impounded water is suddenly released. Authorities and scientists are continuing to evaluate the size, location and stability of any potential blockage,” says ICIMOD.
“The same site devastated by last year’s Rasuwa flood is once again under threat,” said Qianggong Zhang, head of climate and environmental risks at ICIMOD. “An ice avalanche in the Lhende Khola is the suspected trigger, but the cause remains unconfirmed. With a blockage still lodged upstream on the Nepal-China border, authorities warn a second flood is imminent and evacuation orders are now in place for Jilong port,” added Zhang.
Seismic signals
“Preliminary analysis of seismic records has identified unusual signals recorded during the period of the disaster,” says ICIMOD. “Data from a seismometer in Jilong, approximately 12km from the affected area showed anomalous signals, while a second station in Zhangmu also recorded unusual activity,” it adds.
Scientists are investigating whether these signals are connected to the suspected avalanche but no causal link has yet been established.
“The increasing frequency of cryosphere hazards is becoming more visible than ever before. The pace of change is so rapid that current efforts are struggling to keep up,” said Mohd. Farooq Azam, senior cryosphere specialist at ICIMOD. “Today’s disaster in the Lhende Khola underscores the need for stronger regional collaboration, with governments taking coordinated action to respond to cross-border hazards,” Azam continued.